1. Technical Field
The present invention relates to the method of making hollow articles by the process of thermal spraying molten metal particles. More particularly, the process relates to the use of a plastic mandrel that is melted during the thermal spraying process.
2. Discussion of the Prior Art
Valve seats inserts (valve seats) have been used to strengthen the physical characteristics of certain parts of a component, particularly components in internal combustion engines. For example, steel alloy valve seats are used extensively in aluminum engine heads and in some high-performance or alternative fuel cast iron engine heads. The list of enhanced high-performance characteristics desired at the valve seat is quite long, including increased ambient and high temperature wear resistance, higher creep resistance, higher thermal fatigue strength, (under repeated valve impact loading), good thermal conductivity, better corrosion resistance, lower manufacturing costs, and capability of being tribologically compatible with valve materials engaging the valve insert.
A common manufacturing approach that attempts to attain these characteristics is to make the valve seats by powder metallurgy processes which involve several steps: weighing and blending of selected powder mixtures; compaction and green body formation in molds and dies; sintering and sometimes copper infiltration of the compact at respectively 1500.degree. C. and 1080.degree. C.; controlled cooling; post-sintering tempering heat treatment; and finally machining to the desired seat dimensions. This obviously is an involved process which adds considerable cost. To achieve the desired physical characteristics, chemical additions are made to the powder mixture of carbon, chromium, molybdenum (for wear resistance), cobalt and nickel (for heat resistance), and other additions to obtain better thermal conductivity or better self-lubrication. In ferrous based powder mixtures, the resulting product may have its matrix consist of pearlite, bainite or tempered martensite depending on the heat treatment used during compacting and sintering. The sintered insert will always have the same chemistry as the starting green compact with its micro-structure dependent on the heat treatment employed.
To obtain more optimum physical characteristics for the valve seats, very high concentration of certain additions (i.e. 15-25% wt. Cobalt; up to 20% wt. Pb) may be necessary, as well as the introduction of certain chemical ingredients, such as rare earths, which, unfortunately, inhibit or prevent sintering by powder metallurgy techniques. moreover, powder metallurgy does not allow the introduction of low cost oxides or ceramics during processing; ceramics are very useful to achieve certain of the physical characteristics.
When an engine is run with alternate fuels such as natural gas or alcohol, powder metal valve seats for internal combustion engine heads are often inadequate. Powder metal valve seats, when used for intake valve seats in alternate-fuel engines, often contain too little self-lubricant, such as molybdenum disulfide or lead, and thus prematurely wear. Lead is also undesirable as an embedded self-lubricant since it can foul catalytic surfaces used in treating emissions. Moreover lead processing poses environmental concerns and is regulated in the United States.
Commonly assigned U.S. patent application Ser. No. 08/999247, titled METHOD OF MAKING SPRAY-FORMED INSERTS, filed Dec. 29, 1997, now U.S. Pat. No. 5,983,495, and incorporated herein by reference, teaches a method of making valve seats by applying a bulk material to a rotating hollow mandrel. The mandrel was aluminum or other material that did not melt from the heat of the thermal spray process. While this process produced acceptable valve seats, it required removing the insert from the mandrel or machining the mandrel from within the valve seat. Because the mandrel and valve seats were made from two dissimilar metals, the machining step required a complicated bimetallic machining process.
The present invention improves upon this method by using a polymeric mandrel that is melted during the thermal spray process. This eliminates the steps of removing the valve seat from mandrel and the bimetallic machining.
These and other problems of the related art are overcome by the present invention.